My truck is a 78 f250 and it won't keep the battery charged. I was suspicious of the alternator wiring and everywhere I look I find the same info. Haynes and Chilton manuals along with what Napa had on their computers. They don't have any wiring diagrams that match my truck.

Schematics I've looked at:

alternator Bat post ---> black wire to Battery with yellow wire tied off to (A+) tab on the regulator

Fld post------> orange wire to (f) tab on the regulator

Sta post-----> white wire to (s) tab on the regulator

The way it's wired up on my truck:

alternator Bat post ---> black wire to Battery with yellow wire tied off to (A+) tab on the regulator

Fld post------> orange wire to (f) tab on the regulator

Sta post-----> white wire runs through a 2 wire plug and continues into the cab, I assume to the factory ammeter on the dash.

The second wire on the plug is a green wire that runs into the (s) tab on the regulator.

Nothing on the (i) tab on the regulator (schematic says there should be a red wire to the ignition switch)

Put a new battery in the truck yesterday and it holds a constand 12.1V. I was just messing around with the truck again and it's down to 11.8v. I disconnected the battery so it doesn't kill it. If I leave it hooked up for a week without starting it the battery will be completely dead. I have a new voltage regulator in the truck and Checker bench checked the alternator as good. Friday I'm going to drive the truck to Napa and see if they can do a check on the truck while it's running. I'm not sure what to do with it.

The first thing to do is measure the battery voltage with the truck running. If it's 12v or less, it's not charging. It should be around 14v. You hinted around that you may have already checked this.

If it's not charging, the first thing to do is measure the voltage on the bat terminal of the alternator with the engine not running. You should have 12v. If you don't, the fusible link is blown in this wire.

12.3v between the battery post on the alternator and the negative post on the battery while the trucks off.

I have an electric choke on my truck and the diagram shows the white wire from the Sta post on the alternator running to the choke. There is a different wire that someone ran to the electric choke on my truck. The white wire that runs from the Sta post on the alternator goes into the cab somewhere. Should I tap off of that for my choke?

If the choke is working, I would not worry about that right now. But you should not have a wire going into the cab from the sta terminal on the alternator. You should only have a green/red going into the cab and that hooks to the "s" terminal on the regulator.

I would disconnect any wires going to the "s" or "sta" terminal on the alternator.

I'm assuming I'm suppose to have power off the sta post on the alternator at some point. I checked it with the key off and nothing, I tried it in accessory and nothing then checked it after I started the truck and nothing.

One more test. Take the wire off the "f' terminal on the alternator and tape it. Take a scrap piece of wire and put it on the "f" terminal of the alternator, and lay it near the battery. Get the engine started, and put your meter on the battery terminals. Then touch the scrap "f" wire to the bat + terminal. The voltage should jump up, and you may hear the alternator start whining and the engine load down a little bit. That means the alternator is good, and I would replace the regulator. Putting 12v on the "f" terminal throws the alternator into wide open charge.

Sorry to hijack this thread - Franklin2, when you do this test, how high should one see the voltage go? I'd think that if you power the field too long, voltage will increase to the point that it fries the ignition module or boils the battery. I assume one shouldn't try this for too long, is that correct? I've never tried this test before.

That did it. Volts started climbing instantly from low 12's up to 15 as soon as I connected the wire directly from the fld post to the + battery side. Does that mean the voltage regulator is bad? I replaced it about a year ago.

The voltage regulator is really the only thing left. It's what controls the field voltage. Since the test worked, then apparently everything is good except you are not getting field voltage to the alternator, so I would try another.

Make sure the field wire from the regulator to the alternator is in good shape with no bare wires. If the field wire gets shorted to ground for an instant, that's all it takes to blow out the regulator.

Quote:

Sorry to hijack this thread - Franklin2, when you do this test, how high should one see the voltage go? I'd think that if you power the field too long, voltage will increase to the point that it fries the ignition module or boils the battery. I assume one shouldn't try this for too long, is that correct? I've never tried this test before.

The battery will try to hold the voltage down. It usually does hold it down below 16v. The higher the rpm's, the more output the alternator has, so this should be done at idle rpm's. You are correct, it's only a short test that should be done for a few seconds, not minutes. If it were held on to long(over a minute or two), the battery will start getting hot and start boiling, since it's sinking most of the extra power.

That's why I always preach never try the old trick of pulling the battery cable off while the truck is running to see if the alternator is working. The battery is your safe guard against voltage spikes and over voltage, and that is a sure way to blow a module or a computer or radio out.

I pulled the voltage regulator off and and heard something rattling inside so I opened it up and found the tab broken off the "S" part. I bought the regulator new about 6 months ago because the truck was overcharging so I didn't suspect it to be bad. I inspected the orange field wire and it looks good.

Yesterday when I checked for power on the "S" terminal I unpluged the regulator plug and shot the wire which was good. From the "S" terminal where does the power go to from there?

My truck doesn't utilize the "I" terminal. Is this the wrong regulator for my truck since it has an "I" terminal or is it just not utilized?

I looked at the diagram you posted earlier and the green/red wire goes from the "s" terminal to the ignition switch in the cab. A couple times yesterday I tried to start the truck and I heard something under the hood make a light click noise and I switched the key on and the truck wouldn't start. I checked the battery and the starter solenoid to make sure stuff was together good and then it fired up. The starter switch in the cab is keyed differently than the doors.

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